1. Field
The present invention relates generally to radiation therapy, and more particularly to calibrating and/or verification systems used in conjunction with such therapy.
2. Description
According to conventional radiation therapy, a radiation beam is directed toward a tumor located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the tumor according to an established therapy plan. The delivered radiation kills cells of the tumor by causing ionizations within the cells.
Radiation therapy plans are designed to maximize radiation delivered to a target while minimizing radiation delivered to healthy tissue. These goals may not be achieved if the radiation is not delivered exactly as required by the therapy plan. More specifically, errors in radiation delivery can result in low irradiation of tumors and high irradiation of sensitive healthy tissue. The potential for mis-irradiation increases with increased delivery errors.
Radiation may be incorrectly delivered if characteristics of the radiation beam do not match beam characteristics on which the plan is based. In this regard, a radiation therapy plan is designed in view of expected characteristics of the radiation beam that will be used to deliver the therapeutic radiation. These characteristics include particular values of flatness, symmetry and penumbra. Other parameters on which a plan may be based include the divergence of the radiation beam, the distance over which the beam will travel to the therapy area, and attenuative properties of organs and other internal patient structures surrounding the therapy area.
FIG. 1a shows a profile of a radiation field produced by a radiation beam. The field comprises the intersection of the radiation beam with a plane such as a surface of a radiation imaging device. Profile 1 represents variations in the intensity of the radiation field over a central axis of the radiation field. Profile 1 may be used to determine the flatness of the radiation beam. In one example, the flatness is defined as a percentage equal to |(Imax−Imin)/(Imax+Imin)|*100.
Profile 1 may also be used to determine the penumbra of the beam. The left penumbra and the right penumbra may be defined, respectively, as the distance between the 80% intensity level and the 20% intensity level on the left and right sides of the central axis. As shown, the intensity level at the center of the central axis is normalized to 100%.
The values of profile 1 shown in FIG. 1b may be used to determine a symmetry of the radiation beam. The symmetry may also be expressed as a percentage. According to some systems, the symmetry is equal to [(A1+A2+ . . . An)/n−(B1+B2+ . . . Bn)/n]/[(A1+A2+ . . . An)/n−(B1+B2+ . . . Bn)/n]/2*100.
Since a therapy plan may be based on expected values of beam characteristics, these characteristics are often verified prior to delivering radiation according to the plan. Conventional verification systems use a scanning ion chamber to receive a radiation beam and to record intensities at various points of a radiation field produced by the beam. The beam characteristics are computed based on the intensities as described above and verified against expected values. Such systems can be cumbersome, time-consuming and/or otherwise inefficient.
Other beam verification systems acquire an image of the radiation field using a conventional imaging device and determine the intensities at various points of the field based on the image. However, variations in the determined intensities may result from both variations in the beam and differences in the sensitivities of the imaging elements of the imaging device. Accordingly, beam characteristics that are determined based on the intensities do not reliably reflect actual characteristics of the beam.
It would therefore be beneficial to provide a system that may offer more efficient and accurate determination of beam characteristics.